Organic luminescent display device having a semiconductor with an amorphous silicon layer

ABSTRACT

A display device includes a plurality of light emitting elements arranged in a matrix. A scan signal is made to flow into a gate signal line and a data signal is made to flow into a source signal line so that the data signal is applied to a source electrode and the scan signal is supplied to a gate electrode of a control TFT arranged at a portion where the both signal lines intersect when viewed from above. Thus, when the control TFT is turned ON, a drive TFT having a gate electrode connected to the drain electrode is turned ON, so that current is supplied from a power supply line via the source electrode and the drain electrode of the drive TFT to an organic EL element and the organic EL element emits light. A holding capacity is present between the control TFT and the drive TFT. Even when the scan signal becomes LOW level and the control TFT turns OFF, the gate potential of the drive TFT is held for a predetermined period of time by the holding capacity and the organic EL element continues to emit light.

CROSS REFERENCE AND RELATED INFORMATION

This application is a divisional of U.S. application Ser. No.10/500,143, filed Jul. 8, 2004, which is a 371 of PCT/JP03/05222, filedApr. 23, 2003, and is based upon and claims the benefit of priority fromJapanese applications 2002-173816, filed Jun. 14, 2002, 2002-173817,filed Jun. 14, 2002, 2002-125592, filed Apr. 26, 2002 and 2002-159124filed May 31, 2002, the entire contents of which are incorporated hereinby reference.

TECHNICAL FIELD

The present invention relates to a display device having light-emittingelements arrayed in a matrix-like pattern.

BACKGROUND ART

Nowadays, LCDs are widely used as flat panel displays in variousappliances ranging from cellular phones to large-screen televisionmonitors. However, since LCDs are not self-illuminating, they sufferfrom narrow viewing angles, and require a light source such as abacklight, which makes it impossible to reduce power consumption beyonda certain limit. To avoid these inconveniences, as alternatives to LCDs,self-illuminating display devices exploiting, for example, organicelectroluminescence (hereinafter referred to as organic EL) are beingstudied.

This type of display device has, as pixels, organic EL elements arrangedin a matrix-like pattern, and achieves image display by driving thoseorganic EL elements to emit light individually. In a case where they aredriven by an active matrix method, a thin-film transistor (hereinafterreferred to as TFT) is formed in each pixel so that each pixel can bedriven independently. This makes it possible to obtain high-definition,high-brightness display. In addition, it is also possible to obtainhigh-efficient light emission characteristics and thereby reduce powerconsumption. In this type of display device, for each pixel, there areprovided an organic EL element that is composed of a light-emittinglayer sandwiched between two electrodes, a drive TFT that feeds acurrent to one of the electrodes of the organic EL element, and acontrol TFT that controls the operation of the drive TFT. Typically,these drive and control TFTs are formed as polysilicon TFTs having apolycrystalline active layer.

Inconveniently, however, with the drive and control TFTs formed aspolysilicon TFTs, the display device needs to be fabricated through acomplicated, difficult fabrication process, and requires sophisticatedfabrication technology and expensive fabrication equipment. This makesthe display device as an end product accordingly expensive. Moreover,since it is difficult to make the active layer uniformlypolycrystalline, it is difficult to fabricate large-area TFTs withuniform characteristics. This makes it difficult to fabricate alarge-screen display device.

DISCLOSURE OF THE INVENTION

An object of the present invention is to provide a self-illuminatingdisplay device that permits easy fabrication of TFTs and that is suitedfor large-screen applications.

To achieve the above object, according to the present invention, in adisplay device having a plurality of pixels arrayed so as to form amatrix-like pattern, the display device comprises a light-emittingelement that is formed in each pixel, a drive TFT that is formed in eachpixel and that feeds a current to the light-emitting element to make thelight-emitting element emit light, and a control TFT that controls theoperation of the drive TFT. Here, the drive TFT and the control TFT havea semiconductor layer formed of amorphous silicon.

According to the present invention, in the display device describedabove, the light-emitting element is formed in a longitudinally oblongshape, and the drive TFT is formed in a laterally oblong shape and isarranged with the length direction thereof perpendicular to the lengthdirection of the light-emitting element.

According to the present invention, in the display device describedabove, the light-emitting element is formed in a longitudinally oblongshape, the drive thin-film transistor is formed in a laterally oblongshape, a gate signal line and a source signal line connecting to thecontrol thin-film transistor are arranged so as to form a grid-likepattern, the light-emitting element is arranged with the lengthdirection thereof parallel to the source signal line, and the drivethin-film transistor is arranged with the length direction thereofparallel to the gate signal line.

According to the present invention, in the display device describedabove, the drive thin-film transistor has a channel region formed in anelongate shape and is arranged with the length direction of the channelregion thereof parallel to the gate signal line.

According to the present invention, in the display device describedabove, of the source electrode and the drain electrode of the drive TFT,one is formed in a rectilinear shape and the other is formed in a shapesurrounding the one.

According to the present invention, in the display device describedabove, the drive TFT has a U-shaped source electrode and a drainelectrode located between two fork-like portions of the U-shaped sourceelectrode.

According to the present invention, in the display device describedabove, for each row of the matrix-like pattern are formed a gate signalline that is connected to the gate electrodes of all the controlthin-film transistors in the pixels located in the row and a power feedline from which a current is fed via the drive thin-film transistors tothe light-emitting elements in the row, for each column of thematrix-like pattern is formed a source signal line that is connected tothe source electrodes of all the control thin-film transistors in thepixels located in the column and that crosses the gate signal line, andwithin each area surrounded by gate signal lines and source signallines, the light-emitting element, the drive TFT, the power feed line,and the control TFT are arranged in this order along the source signalline as seen in a plan view.

According to the present invention, in the display device describedabove, between the drive TFT and the control TFT is formed a holdingcapacitor of which one electrode is shared as the power feed line and ofwhich the other electrode is formed by an auxiliary electrode thatconnects to the drain electrode of the control TFT, and the auxiliaryelectrode is electrically connected to the gate electrode of the driveTFT.

According to the present invention, in the display device describedabove, the display device comprises light-emitting elements that emitlight of different colors, a plurality of power feed lines are formed soas to correspond to light of the different colors, the plurality ofpower feed lines are arranged between the drive thin-film transistor andthe control thin-film transistor, and the light-emitting elements arefed with a current from the corresponding power feed lines.

According to the present invention, in the display device describedabove, the gate signal line is used as the gate electrode of the controlTFT, and the control TFT is formed above the gate signal line.

According to the present invention, in the display device describedabove, a bank layer is arranged around the light-emitting element, thebank layer is formed so as to overlap the drive TFT, a cut is formed inthe bank layer between the light-emitting element and the drive TFT, anda light-shielding film is formed on the bank layer at least in a portionthereof near the cut.

According to the present invention, in the display device describedabove, a bank layer is arranged around the light-emitting element, thebank layer is formed so as to overlap the control TFT, a cut is formedin the bank layer between the light-emitting element and the control TFTformed in the next pixel, and a light-shielding film is formed on thebank layer at least in a portion thereof near the cut.

According to the present invention, in the display device describedabove, a bank layer is formed so as to cover the drive thin-filmtransistor and the control thin-film transistor, the bank layer hasedges thereof located between the drive and control thin-filmtransistors and the light-emitting element, and a light-shielding filmis formed on the bank layer.

According to the present invention, in the display device describedabove, the display device further comprises a pixel electrode that isarranged below a light-emitting layer of the light-emitting element andthat connects to the drive TFT and a common electrode that is arrangedso as to face the pixel electrode with the light-emitting layerinterposed in between and that covers the bank layer, and thelight-shielding film is formed by the common electrode.

According to the present invention, in the display device describedabove, the drive TFT and the control TFT are of an n-channel type.

According to the present invention, in the display device describedabove, the drive TFT and the control TFT are of a p-channel type.

According to the present invention, in the display device describedabove, the light-emitting element is of an organic electroluminescencetype.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a circuit diagram of each pixel of a display device embodyingthe invention.

FIG. 2 is a plan view showing pixels of the display device of theinvention and a portion around it.

FIG. 3 is a schematic cross-sectional view of the light-emitting elementformed within a pixel.

FIG. 4 is a plan view showing one of three, i.e., RGB, pixels.

FIG. 5 is a schematic cross-sectional view showing the control TFT and aportion around it.

FIG. 6 is a schematic cross-sectional view showing the power feed lineand the holding capacitor and a portion around it.

FIG. 7 is a schematic cross-sectional view showing the drive TFT and aportion around it.

FIG. 8 is a diagram showing how light, when shielded and when not, isincident on the TFTs.

BEST MODE FOR CARRYING OUT THE INVENTION

Hereinafter, an embodiment of the present invention will be describedwith reference to the drawings. FIG. 1 is a diagram schematicallyshowing the circuit configuration of a pixel in a display deviceaccording to the invention. FIG. 2 is a plan view showing pixels of thedisplay device and a portion around it. FIG. 3 is a schematiccross-sectional view (taken along line D-D shown in FIG. 2) of thelight-emitting element formed within the pixel. In this embodiment, thelight-emitting element is realized with an organic EL element 1. Itshould be noted that, while a common electrode 33 is shown in FIG. 3, itis omitted in FIG. 2 to make the figure easy to view.

As shown in FIG. 1, the organic EL element 1 emits light when a currentflows therethrough from a pixel electrode 14 to a common electrode 33,and its brightness can be adjusted by controlling the level of thecurrent. The organic EL element 1 of a particular pixel is made to emitlight in the following manner. A scanning signal is fed to a gate signalline 2, and a data signal is fed to a source signal line 3. At theintersection between the two signal lines as seen in a plan view, thereis formed a control TFT 6 as a second transistor, so that the datasignal and the scanning signal are fed to the source electrode 11 andthe gate electrode 13, respectively, of the control TFT 6. When thecontrol TFT 6 is turned on in this way, a drive TFT 5 formed as a firsttransistor and having the gate electrode 10 thereof connected to thedrain electrode 12 of the control TFT 6 is turned on. Thus, a current isfed from a power feed line 4 through the source electrode 8 and thedrain electrode 9 of the drive TFT 5 to the organic EL element 1,causing it to emit light. Between the control TFT 6 and the drive TFT 5,there exists a holding capacitor 34, of which one electrode is formed bythe power feed line 4, and of which the other electrode is formed by anauxiliary electrode that is formed simultaneously with the drainelectrode 12. Thus, when the scanning signal turns low and accordinglythe control TFT 6 is turned off, the gate potential of the drive TFT 5is maintained by the holding capacitor 34, permitting the organic ELelement 1 to keep emitting light.

Next, the structure of the display device of the invention will bedescribed with reference to FIGS. 2 and 3. In the display area of thedisplay device, gate signal lines 2 and source signal lines 3 are laidso as to form a grid-like pattern, and pixels are formed individually inthe areas surrounded by the gate signal lines 2 and the source signallines 3. In each pixel, there is formed an organic EL element 1exploiting organic EL as the light-emitting layer 16, and, along withthis organic EL element 1, there are formed a drive TFT 5 that feeds acurrent from a power feed line 4 to the organic EL element 1 and acontrol TFT 6 that controls the drive TFT 5 by turning it on and off.When a current is fed from a power feed line 4 to an organic EL element1, the light-emitting layer 16 thereof emits light of a given color, andits brightness can be adjusted by controlling the level of the current.

On a glass substrate 30, a plurality of gate signal lines 2 are laidparallel to one another, and, along each gate signal line 2, three powerfeed lines 4 are laid. The gate signal lines 2 and the power feed lines4 are formed simultaneously in a single fabrication step, and are formedof Al or Cr. The three power feed lines 4 are for R, G, and B colors,respectively, and accordingly the power feed line 4R for the R color isconnected to an organic EL element 1 having a red light-emitting layer16(R), the power feed line 4G for the G color is connected to an organicEL element 1 having a green light-emitting layer 16(G), and the powerfeed line 4B for the B color is connected to an organic EL element 1having a blue light-emitting layer 16(B). Organic EL elements 1 havingdifferent light-emitting materials emit light of different colors, andemit light with different light emission efficiency. Thus, by formingdifferent power feed lines 4 for different colors so as to feedappropriate levels of current for the different colors, it is possibleto achieve optimum full-color display.

When the gate signal line 2 and the power feed lines 4 are formed, thegate electrode 10 of the drive TFT 5 is formed simultaneously betweenthe power feed lines 4 and the organic EL element 1. This gate electrode10 is formed in a laterally oblong shape along the power feed lines 4,and has one of the shorter sides thereof formed in a rectilinear shapeand the other in an arc shape. Since the drive TFT 5 serves to feed acurrent to the organic EL element 1, as much current as possible needsto be passed through the drive TFT 5 when it is on, and accordingly itsgate electrode 10 is formed as large as possible.

On top of the glass substrate 30, there is laid a gate insulating film31 formed of SiNx (silicon nitride film), and this gate insulating film31 covers the gate signal line 2 and the power feed lines 4. On top ofthe gate insulating film 31, there is laid an amorphous silicon layer(hereinafter referred to as the a-Si layer), of which unnecessary partsare then removed by photolithography so as to leave only those portionsthereof which correspond to the semiconductor layers (active layers) 7and 13 of the TFTs 5 and 6. Here, the a-Si layer patch 7 left in thedrive TFT 5 has a shape whose contour runs along the outer edge of thegate electrode 10, and lies over the greater part of the gate electrode10, protruding out of it in the shorter-side and arc-shaped portionsthereof. On the other hand, the a-Si layer patch 13 left in the controlTFT 6 has a rectangular shape, and lies across the gate signal line 2.

On top of the a-Si layer patches 7 and 13 and the gate insulating film31, there is formed a metal layer having layers of Al and Mo laid on oneanother, and then this metal layer is patterned by photolithography toform the source signal line 3, the source and drain electrodes of theTFTs 5 and 6, and other elements. Here, the source signal line 3 isformed perpendicular to the gate signal line 2, with the sourceelectrode 11 protruding from the source signal line 3, near theintersection between the source and gate signal lines 3 and 2, so as toreach above the a-Si layer patch 13 of the control TFT 6. The drainelectrode 12 of the control TFT 6 connects to the gate electrode 10 ofthe drive TFT 5 via an auxiliary electrode 134 and a transparentelectrode 21, which will be described later. Thus, when the control TFT6 is turned on, it feeds the current flowing through the source signalline 3 to the gate electrode 10 of the drive TFT 5. The auxiliaryelectrode 134, which connects to the drain electrode 12 of the controlTFT 6, covers the power feed lines 4 with the gate insulating film 31laid in between, and the power feed lines 4 and the auxiliary electrode134 together form the holding capacitor 34. In a-Si TFTs, the gateinsulating film 31 is significantly thicker than in polysilicon TFTs,and this makes the capacitance of the holding capacitor 34 low. To avoidshortage of capacitance here, the auxiliary electrode 134 is required tocover as wide an area as possible over the power feed lines 4, andtherefore the auxiliary electrode 134 is formed so as to cover thegreater part of the power feed lines 4 within each pixel.

Within the drive TFT 5, there are formed a substantially U-shaped sourceelectrode 8 and a substantially rectilinear drain electrode 9 locatedbetween the two fork-like portions of the source electrode 8. From anouter-edge portion of the source electrode 8 facing away from the drainelectrode 9 protrudes an electrode 8 a that reach near the power feedlines 4 so as to be connected, via a transparent electrode 19, whichwill be described later, to whichever of the power feed lines 4corresponds to the color of the pixel. On the other hand, from the drainelectrode 9 protrudes an electrode 9 a that, just outside the a-Si layerpatch 7, turns toward the organic EL element 1 so as to reach andelectrically connected to the pixel electrode 14 of the organic ELelement 1.

The outer edge of the source electrode 8 of the drive TFT 5 is so shapedas to run along the outer edge of the gate electrode 10, and the twofork-like portions of the U shape are formed as long as possible abovethe gate electrode 10. The drain electrode 9 is formed elongate so as tobe shaped like the two fork-like portions of the source electrode 8. Thedrive TFT 5 serves to feed a current from the power feed line 4 to thepixel electrode 14, and therefore as much current as possible needs tobe passed through the drive TFT 5 when it is on. A a-Si TFT is moredifficult for a current to pass through than a polysilicon TFT.Accordingly, when an a-Si TFT is used as the drive TFT 5, the drive TFT5 needs to be formed as large as possible. Specifically, more currentcan be passed by reducing the channel length and increasing the channelwidth. However, since it is technically infeasible to reduce the channellength over a certain limit, an effective way here is to form the driveTFT 5 as large as possible so as to increase the channel width.Accordingly, in this embodiment, the shapes of the source and drainelectrodes 8 and 9 are so designed that the drive TFT 5 permits as muchcurrent as possible to pass therethrough. Specifically, the gateelectrode 10 of the drive TFT 5 is formed in a laterally oblong shape,and the source and drain electrodes 8 and 9 are formed elongate. Thismakes it possible to secure a large channel width within a limitedspace. In particular, forming the laterally oblong gate electrode 10with its length direction parallel to the gate signal line 2 makes itpossible to form the drive TFT 5 over the entire interval betweenadjacent source signal lines 3. Moreover, aligning the direction of thechannel width parallel to the gate signal line 2 makes it possible toeffectively increase the channel width within the limited size of thedrive TFT 5. Furthermore, forming the source electrode 8 in a U shapeand arranging the drain electrode 9 between the two fork-like portionsof the U shape permits the source electrode 8 to be located on bothsides of the drain electrode 9 and thus helps double the channel width.This makes it possible to effectively obtain a large channel widthwithin a small space.

The control TFT 6 simply serves to control the drive TFT by turning iton and off, and therefore, as opposed to the drive TFT 5, less voltageneeds to be passed through the control TFT 6. This permits the controlTFT 6 to be formed in an accordingly small size. Making the control TFT6 smaller helps secure an accordingly large space for the drive TFT 5,and thus helps make the TFT 5 larger. Hence, a branch of the sourcesignal line 3 is formed that diverges therefrom near the intersectionbetween the gate signal line 2 and the source signal line 3 as seen in aplan view, and the far end of the branch is used as the source electrode11 of the light-emitting layer 16. Moreover, as will be described later,by laying the source signal line 3 and its branch, namely the sourceelectrode 11, above the gate signal line 2 as seen spatially, and byforming the drain electrode 12 of the control TFT 6 simultaneously withand in structurally the same layer as the source electrode 11, it ispossible, conveniently, to share the gate signal line 2 as the gateelectrode 13 of the control TFT 6.

In the control TFT 6, the source electrode 11 and the drain electrode 12are simply so arranged that one side of the former faces one side of thelatter on the a-Si layer patch 13. By contrast, in the drive TFT 5, thesource electrode 8 is so arranged as to surround the drain electrode 9.This results in an accordingly large channel width. Moreover, in thisembodiment, in the drive TFT 5, the length over which the drainelectrode 9 faces the source electrode 8 is three times the channelwidth of the control TFT 6 or more, and thus the channel width of thedrive TFT 5 is six times the channel width of the control TFT 6 or more.By securing a large channel width in the drive TFT 5 in this way, it isin practice possible to realize optimum display even in a case where ana-Si TFT is used as the drive TFT. In this embodiment, the drive TFT 5is formed as large as possible, with the result that the channel widthof the drive TFT 5 is six times the channel width of the control TFT 6.It is, however, possible to obtain high-quality display by making thechannel width of the drive TFT 5 equal to four times the channel widthof the control TFT 6 or more. In this embodiment, the channel lengths ofthe control TFT 6 and the drive TFT 5 are made approximately equal. Itis, however, possible to make the channel length of the drive TFT 5smaller than the channel length of the control TFT 6 to make itaccordingly easier for a current to pass through the former.

An insulating film 32 formed of SiNx is formed so as to cover the sourcesignal line 3 and the TFTs 5 and 6, and on top of the insulating film 32is laid a transparent electrode formed of ITO (indium tin oxide) or IZO(indium zinc oxide). This transparent electrode is patterned byphotolithography to form the pixel electrode 14. This pixel electrode 14is located within each pixel, has a substantially elliptic shape, isarranged along the source signal line 3, and has a portion thereofprotruding so as to overlap a portion of the drain electrode 9 a of thedrive TFT 5. In this portion where the pixel electrode 14 overlaps thedrain electrode 9 a, a contact hole 23 is formed in the insulating film32 above the drain electrode 9 a, and the pixel electrode 14 iselectrically connected to the drain electrode 9 a via the contact hole23.

When the pixel electrode 14 is formed, a patch of the transparentelectrode is left between the power feed lines 4 and the sourceelectrode 8 a of the drive TFT 5 so that one of the power feed lines 4and the source electrode 8 a are electrically connected together.Specifically, above whichever of the power feed lines 4 corresponds tothe pixel, a contact hole 18 a is formed in the gate insulating film 31and the insulating film 32 so that part of the power feed line 4 isexposed; above the source electrode 8 a of the drive TFT 5, a contacthole 18 b is formed in the insulating film 32 so that part of the sourceelectrode 8 a is exposed; and the transparent electrode patch 19 makescontact with the parts of the power feed line 4 and the source electrode8 a thus exposed via those contact holes 18 a and 18 b.

Another patch of the transparent electrode is left between the auxiliaryelectrode 134 and the gate electrode 10 of the drive TFT 5, and thistransparent electrode patch 21 makes contact with parts of the auxiliaryelectrode 134 and the gate electrode 10 exposed via contact holes 20 aand 20 b, and thereby electrically connects those electrodes 10 and 134together.

FIG. 4 is a plan view of one of the three pixels for the R, G, and Bcolors shown in FIG. 2, and now a description will be given of the crosssections of the layers taken in different portions of the pixel. FIG. 5is a schematic cross-sectional view (taken along line A-A shown in FIG.4) showing the control TFT 6 and a portion around it. First, on theglass substrate 30, which is common to the entire display device, thegate signal line 2 is formed. On top, the gate insulating film 31 formedof SiNx is formed, and thus the gate signal line 2 also issimultaneously covered by the gate insulating film 31. Further on top ofthe gate insulating film 31, the a-Si layer 13 is formed above the gatesignal line 2 so as to lie over it. On top of the a-Si layer 13, themetal layer having layers of Al and Mo laid on one another is formed,with an N-type a-Si thin film 13 a containing an N-type impurity laid inbetween. This metal layer is then patterned by photolithography to formthe source signal line 3, the source electrode 11 diverging therefrom,and the drain electrode 12. Further on top are laid, one on top ofanother, the insulating film 32 formed of SiNx, a protection layer 15formed of SiO₂ (silicon oxide), a bank layer 17, and the commonelectrode 33.

As described earlier, the three power feed lines 4 are formed so as tocorrespond to R, G, and B pixels, respectively. To minimize thenarrowing, resulting from the provision of as many as three power feedlines 4, of the area that can be used for the organic EL element 1, thepower feed lines 4 are laid parallel to the gate signal line 2, and theythemselves are used to spatially form the holding capacitor 34 abovethem. This eliminates the need to form an extra holding capacitor lineor to secure an extra flat area for the formation of the holdingcapacitor 34. Conventionally, the holding capacitor 34 is formed bylaying a holding capacitor line in such a way that it, like the gatesignal line 2 and the source signal line 3, runs through each pixel. Inthis embodiment, by contrast, there is no need to do so.

Now, the specific structure of the power feed lines 4 and the holdingcapacitor 34 will be described with reference to FIG. 6, which is aschematic cross-sectional view (taken along line B-B shown in FIG. 4) ofthe power feed lines and the holding capacitor and a portion aroundthem. First, on top of the glass substrate 30, which is common to theentire display device, in the same layer as the gate signal line 2 shownin FIG. 5, the B power feed line 4B, the G power feed line 4G, and the Rpower feed line 4R are formed, which serve as one electrode of theholding capacitor 34. On top, the gate insulating film 31 formed of SiNxis formed, and thus the three power feed lines 4 also are simultaneouslycovered by the gate insulating film 31. Further on top of the gateinsulating film 31, in the same layer as the source electrode 11 and thedrain electrode 12 shown in FIG. 5, the metal layer having layers of Aland Mo laid on one another is formed, and this metal layer is patternedby photolithography so that the auxiliary electrode 134, i.e., the otherelectrode of the holding capacitor 34, is formed as an extension fromthe drain electrode 12. The power feed lines 4 and the holding capacitor34 thus formed serve, more specifically, as individual holdingcapacitors 34 a, 34 b, and 34 c (FIG. 1) necessary for pixels of thedifferent colors.

The holding capacitor 34 has the auxiliary electrode 134 thereofconnected to the gate electrode 10 (FIG. 2) of the drive TFT 5.Specifically, in the insulating film 32 above the auxiliary electrode134 of the holding capacitor 34, the contact hole 20 a is formed so thatpart of the auxiliary electrode 134 is exposed. Moreover, as shown inFIG. 2, the contact hole 20 b also is formed in part of the gateinsulating film 31 and the insulating film 32 so that part of the gateelectrode 10 is exposed. Then, the transparent electrode 21 formed ofITO or IZO is formed to lie over the two contact holes 20 a and 20 b,and thus the auxiliary electrode 134 exposed via the contact hole 20 aand the gate electrode 10 exposed via the contact hole 21 b areelectrically connected together via the transparent electrode 21.Further on top are laid, one on top of another, the protection layer 15,the bank layer 17, and the common electrode 33.

The structure of the drive TFT 5 is shown in FIG. 7, which is aschematic cross-sectional view (taken along line C-C shown in FIG. 4) ofthe drive TFT and a portion around it. First, on the glass substrate 30,which is common to the entire display device, the gate electrode 10 isformed. On top, the gate insulating film 31 formed of SiNx is formed,and thus the gate electrode 10 also is simultaneously covered by thegate insulating film 31. Further on top of the gate insulating film 31,the a-Si layer patch 7 is laid as a semiconductor layer. On top of thisa-Si layer patch 7, the metal layer having layers of Al and Mo laid onone another is formed, with an N-type a-Si thin film 7 a containing anN-type impurity laid in between. This metal layer is then patterned byphotolithography to form electrodes that will be used as the U-shapedsource electrode 8 and the drain electrode 9. Further on top, theinsulating film 32 formed of SiNx is formed.

With the elements and lines formed as described above, the organic ELelement 1 is driven to emit light. Now, the structure of the organic ELelement 1 will be described with reference to FIG. 3. Reference numeral15 represents the protection film formed of SiO₂, which is formed on topof the insulating film 32 and overlaps an edge portion of the pixelelectrode 14 of the organic EL element 1. That is, the protection layer15 covers the edge portion of the pixel electrode 14, but is removedabove the greater part, including a central portion, of the pixelelectrode 14. Reference numeral 17 represents the bank layer formed ontop of the protection layer 15 and formed of novolac resin, and thisbank layer is formed thicker than the protection layer 15 or theinsulating film 32. Organic EL as a light-emitting material is appliedin the area surrounded by the bank layer 17, and therefore the banklayer 17 is so formed as to run along the outer edge of the pixelelectrode 14 and surround the pixel electrode 14. For the sole purposeof keeping the light-emitting material in position, the bank layer 17has only to be formed around the pixel electrode 14. In this embodiment,however, the bank layer 17 is formed also above the two TFTs 5 and 6 andthe power feed lines 4. The bank layer 17 may be formed of any organicor inorganic resin other than novolac resin so long as it is aninsulating material.

A light-emitting material corresponding to the color of each pixel issprayed onto the pixel electrode 14 by ink jet so as to be held insidethe area surrounded by the bank layer 17. This light-emitting materialis organic EL, such as a conjugate high-polymer precursor. Thereafter,through heating, the light-emitting material is polymerized, so that R,G, and B light-emitting layers 16 are formed in individual pixels.

Reference numeral 33 represents the common electrode formed of Al or Cr,which is laid on top of the light-emitting layer 16. The commonelectrode 33 is formed over the entire display area, and is fed with apredetermined voltage. Forming the common electrode 33 as a metal layerpermits the light-emitting layer 16 to emit light, and therefore thecommon electrode 33 may be formed of any metal other than Al or Cr.However, forming the common electrode 33 as a layer of a metal with ahigh light reflectivity such as Al or Cr as in this embodiment makes itpossible to efficiently use the light from the light-emitting layer 16for display and thereby achieve display with higher brightness. When acurrent higher than a threshold level is fed to the pixel electrode 14,the light-emitting layer 16 emits light, and this light can be observedfrom the side of the glass substrate 30.

For example, in a case where +8V (Vdd(R) and Vdd(G)) are fed to the Rand G power feed lines 4, +10V (Vdd(B)) is fed to the B power feed line4, and −3V is fed to the common electrode 33, when a scanning signal isfed to the gate signal line 2 and a data signal is fed to the sourcesignal line 3, the scanned control TFT 6 turns on, and the data signalthat flows through the source signal line 3 at that moment is fedthrough the drain electrode 12 of the control TFT 6 to the gateelectrode 10 of the drive TFT 5. This turns the drive TFT 5 on.Thereafter, even when the control TFT 6 turns off, the holding capacitor34 holds the drive TFT 5 on, and thus the current that flows through thecorresponding power feed line 4 is fed through the drive TFT 5 to thepixel electrode 14. Thus, a potential difference greater than apredetermined level appears between the pixel electrode 14 and thecommon electrode 33, and thus a current flows through the light-emittinglayer 16, resulting in emission of light of the color corresponding tothe light-emitting material. Incidentally, in organic EL, a bluelight-emitting material exhibits lower light emission efficiency thanlight-emitting materials of other colors, and therefore the pixelelectrode 14 of a blue pixel is fed with a higher voltage than the pixelelectrodes 14 of other pixels.

According to the present invention, the light-emitting layer 16 isformed in a longitudinally oblong shape and is arranged parallel to thesource signal line 3, the drive TFT 5 is formed in a laterally oblongshape and is arranged parallel to the gate signal line 2. That is, thedrive TFT 5 is arranged in such a way that the length direction of thedrive TFT 5 is perpendicular to the length direction of thelight-emitting layer 16. This arrangement permits to arrange, within thelimited area surrounded by the source signal lines 3 and the gate signallines 2, a large light-emitting layer 16 while making the drive TFT 5 aslarge as possible. In particular, it is possible to form the drive TFT 5close to the source signal line 3, and thus to arrange the drive TFT 5over the entire interval between adjacent source signal lines 3. Thishelps form the drive TFT 5 large. Thus, even when an a-Si TFT is used asthe drive TFT 5, it is possible to feed a sufficient current to thelight-emitting layer 16 and thereby obtain optimum display.

Here, the purpose of arranging the drive TFT 5 having a laterally oblongshape perpendicular to the light-emitting layer 16 having alongitudinally oblong shape is to permit the drive TFT 5 to pass asufficient current, i.e., to increase the channel width. Accordingly, inthe drive TFT 5, by forming the channel elongate and arranging it withthe channel width direction perpendicular to the length direction of thelight-emitting layer 16, it is possible to effectively increase thechannel width within the limited area.

Moreover, according to the present invention, in the area surrounded bythe gate signal lines 2 and the source signal lines 3, thelight-emitting layer 16, the drive TFT 5, the three power feed lines 4,and the control TFT 6 are arranged in this order along the source signalline 3. This arrangement permits the individual elements to be arrangedneatly, and also helps reduce the area needed for the arrangement ofelements other than the light-emitting element and shorten the currentpath length from the power feed line to the light-emitting element.Moreover, by arranging the power feed lines 4 between the drive TFT 5and the control TFT 6 so that the power feed lines 4 can be shared asthe holding capacitor 34 for the drive TFT 5, it is possible toefficiently use the space within the pixel, and to arrange a pluralityof power feed lines 4 corresponding to light-emitting layers 16 ofdifferent colors.

Composed of various layers laid one on top of another as describedabove, the pixel includes many near-transparent layers such as the gateinsulating film 31, the insulating film 32, the transparent electrode21, the protection layer 15, and the bank layer 17 shown in FIGS. 5 to 7and the pixel electrode 14 shown in FIG. 3. Moreover, as will beunderstood from the description given thus far, as the result of theattempt to minimize the area for the arrangement of control elements andlines and to maximize the light-emitting area, the light-emitting layer16 is located close to the light-emitting layer 16 of the next pixel,and the drive TFT 5 is located close to the light-emitting layer 16within the pixel. In particular, the drive TFT 5 has a large channelwidth, and is arranged parallel to the light-emitting layer 16. Thismakes the light from the light-emitting layer 16 more likely to beincident on the semiconductor layer of the TFT. Light incident on theTFT causes light-induced leakage therein, making it impossible to feed apredetermined current to the light-emitting element. This makes theactual display different from what is expected to be reproducedaccording to the display signal, and thus leads to lower displayquality.

Now, with reference to FIG. 8, how the light from the light-emittinglayer 16 is shielded will be described. FIG. 8(A) is a diagram showinghow the light is incident on the TFT when the light is not shielded, andFIG. 8(B) is a diagram showing the path of the light when the light isshielded. For convenience' sake, in these figures, only the relevantlayers are shown and the other layer are omitted. In FIG. 8(A), thelight emitted from the light-emitting layer 16 covered by the commonelectrode 33 passes through the bank layer 17 and is incident on theunillustrated semiconductor active layer of the drive TFT 5. Here, someof the light is directly incident on the semiconductor active layerthrough the side face thereof, and some of the light is reflected fromthe common electrode 33 and is then incident on the semiconductor activelayer through the top face thereof. Simultaneously, the light isincident also on the unillustrated semiconductor active layer of thecontrol TFT 6. In particular, as will be understood from its use as alight semiconductor, a-Si is liable to be affected by light, andproduces a large leak current when light is incident thereon.

To avoid this, as shown in FIG. 8(B), a cut 35 is formed in the banklayer 17 between the light-emitting layer 16 and the drive TFT 5. Asimilar cut 36 is formed also in the bank layer between thelight-emitting layer 16 and the control TFT 6. Thereafter, the commonelectrode 33 is formed from above so as to cover them. As describedearlier, the common electrode 33 is a layer of a metal such as Al or Crthat reflects light, and the inner surface of the common electrode 33formed so as to cover the cuts 35 and 36 reflects light and therebyprevents it from being incident on the TFTs.

Here, advisably, the inner surface of the common electrode 33 at thecuts 35 and 36 is so shaped as to reflect light in the directionpointing to the bottom of the figure, i.e., toward the unillustratedglass substrate. This helps increase the apparent brightness of thedisplay when it is observed from the side of the glass substrate. Thecuts 35 and 36 are shaped, advisably, as follows. The light-emittinglayer side contours of the cuts 35 and 36 run along the contour of thelight-emitting layer 16. This makes it possible to efficiently use thelight from the light-emitting layer 16 for display. The TFT sidecontours of the cuts 35 and 36 is located as close as possible to theTFTs. This makes it possible to surely prevent incidence of light on theTFTs.

As shown in FIG. 2, whereas the cut 35 located between thelight-emitting layer 16 and the drive TFT 5 is formed in a rectilinearshape in the direction of the width of the pixel, the cut 36 locatedbetween the light-emitting layer 16 and the control TFT 6 is so shapedas to run roughly along the width-direction outer edge of thelight-emitting layer 16. That is, since the drive TFT 5 is an a-Si TFT,it is formed large over the entire width of the pixel so as to becapable of feeding a sufficient current to the pixel electrode 14, and,to prevent incidence of light on this drive TFT 5, the cut 35 is formedlong along the drive TFT 5. On the other hand, since the control TFT 6is formed at the intersection between the gate signal line 2 and thesource signal line 3, the cut 36 is formed at least near theintersection between the two signal lines 2 and 3. The cut 36 is formedalso between adjacent intersections between the gate signal line 2 andthe source signal line 3. This helps surely prevent incidence of lighton the control TFT 6, and also helps direct the light from thelight-emitting layer 16 to a lower portion of the display area. In thisway, it is possible to shield unnecessary light from the light-emittinglayer 16 in such a manner as to enclose the light source, andsimultaneously to permit the light reflected from the cut 36 to be mixedwith the light traveling from the light-emitting layer 16 along thedirect path, resulting in still higher brightness.

The light from the light-emitting layer 16 not only exerts its effectwithin a given pixel but may also affect the drive TFT of the nextpixel. Out of this consideration, and from the viewpoint of increasingthe reflection efficiency at the cuts, the cuts 35 and 36 are advisablyso formed as to have as close a length as possible to the length of theshorter sides of the pixel.

On top of the bank layer 17 that covers the TFTs 5 and 6 is laid thecommon electrode 33. That is, the TFTs 5 and 6 are covered by thelight-shielding common electrode 33 from above. This prevents incidenceof light on the TFTs 5 and 6 from above. Incidentally, in a case wherethe TFTs 5 and 6 are covered by an electrically conductive member suchas the common electrode 33 from above, the bank layer 17 serves also toincrease the distance from the TFTs 5 and 6 to the common electrode 33.Since a predetermined voltage is always applied to the common electrode33, if the common electrode 33 is arranged near the TFTs 5 and 6, itadversely affects the operation of the TFTs 5 and 6. Therefore, it isadvisable to arrange the TFTs 5 and 6 as far away as possible from thecommon electrode 33, and it is possible to secure a sufficient distancethere by giving a greater film thickness to the bank layer 17 coveringthe TFTs 5 and 6. Accordingly, even in a case where no bank layer 17 isformed around the light-emitting element, forming a bank layer 17 abovethe TFTs 5 and 6 and laying the common electrode 33 above the bank layer17 helps prevent incidence of light on the TFTs 5 and 6. Thus, formingthe bank layer 17 above the TFTs 5 and 6 is effective. In this case, byforming the bank layer 17 in such a way that its edge is located betweenthe light-emitting element and the TFTs 5 and 6, it is possible to omitthe cuts 35 and 36.

In this embodiment, the light-shielding film that is laid above the cutsin the bank layer and above the TFTs is formed by the common electrode.This eliminates the need to form a light-shielding film separately fromthe common electrode, and thus helps simplify the fabrication process.However, according to the invention, the light-shielding film does notnecessarily have to be formed by the common electrode, but may be formedby forming a black resin film on the bank layer covering the TFTs.

As described above, the present invention aims at forming an a-Si TFT asa TFT for feeding a current to an organic EL element. This eliminatesthe need to fabricate a polysilicon TFT, and thus helps simplify thefabrication process and thereby produce an inexpensive display device.It is to be understood that the present invention can be implemented inany other manner than specifically described above within the scope ofits concept. For example, although the embodiment described aboveassumes the use of a drive TFT 5 provided with a drain electrode 9 in arectilinear shape and a source electrode 8 in a U shape enclosing thedrain electrode 9 so as to have an elongate channel region along bothsides of the drain electrode 9, the drive TFT 5 may be formed in anyother manner so long as it can feed a sufficient current to the organicEL element; for example, it is possible to use one provided with asource and a drain electrode both in a laterally oblong shape and havingthose source and drain electrodes so arranged that the direction of thechannel width is perpendicular to the length direction of thelight-emitting layer 16. The source and drain electrodes of the driveTFT 5 may be given any other shapes; for example, it is possible to formthe source electrode in a C shape and the drain electrode in arectilinear shape, or the drain electrode in a U shape and the sourceelectrode in a rectilinear shape.

According to the present invention, the TFT that feeds a current to theorganic EL element is formed as an n-channel a-Si TFT. It is, however,also possible to use instead a p-channel a-Si TFT. Forming the TFT withthe same type of channel helps simplify the fabrication process andthereby produce an inexpensive display device.

INDUSTRIAL APPLICABILITY

According to the present invention, in a display device having aplurality of pixels arrayed so as to form a matrix-like pattern, thedisplay device comprises a light-emitting element that is formed in eachpixel, a drive TFT that is formed in each pixel and that feeds a currentto the light-emitting element to make the light-emitting element emitlight, and a control TFT that controls the operation of the drive TFT.Here, the drive TFT and the control TFT have a semiconductor layerformed of a-Si. This makes it possible to fabricate large-area TFTs withuniform characteristics without the need for sophisticated fabricationtechnology or expensive fabrication equipment. Thus, it is possible toprovide a self-illuminating display device that is inexpensive and thatis suited for large-screen applications.

When an a-Si TFT is used, a drive TFT needs to be formed as large aspossible so as to be able to feed a sufficient current to alight-emitting element. By forming the light-emitting element in alongitudinally oblong shape, forming the drive TFT in a laterally oblongshape, arranging the drive TFT so that its length direction isperpendicular to the length direction of the light-emitting element, andarranging the light-emitting element, the drive TFT, a power feed line,and a control TFT in this order along a source signal line, it ispossible to efficiently arrange the individual elements in the limitedspace within a pixel. This makes it possible to secure a large area forthe arrangement of the light-emitting element and simultaneously makethe drive TFT large, and thus helps obtain a display device that offerssatisfactory display quality.

By forming the channel region of the drive TFT elongate, forming one ofthe source and drain electrodes thereof in a substantially rectilinearshape, and forming the other in a shape surrounding the one, it ispossible to increase the channel width of the drive TFT. This makes itpossible to supply a sufficient current to the light-emitting elementeven in a case where an a-Si TFT is used.

By forming a holding capacitor between the drive TFT and the controlTFT, sharing the power feed line as one electrode of the holdingcapacitor, and forming the other electrode thereof with an auxiliaryelectrode that connects to the drain electrode of the control TFT and tothe gate electrode of the drive TFT, it is possible to compactly arrangethe individual elements without the need for a dedicated capacitanceline. This helps enlarge the area that can be secured for thelight-emitting element, and thus contributes to increased light emissionefficiency and brightness.

By forming a plurality of power feed lines corresponding respectively tolight-emitting elements emitting light of different colors, arrangingthe plurality of power feed lines between the drive thin-film transistorand the control thin-film transistor, and feeding the light-emittingelements with currents from the corresponding power feed lines, it ispossible to feed the light-emitting elements for the different colors,which have different light emission efficiency from one another, withappropriate currents and thereby achieve optimum full-color display.

Using a gate signal line as the gate electrode of the control TFT, andforming the control TFT above the gate signal line, it is possible toeliminate the need to separately form a gate electrode. This eliminatesthe need to secure an extra area for the formation of the control TFT,and thus helps secure a large space for the arrangement of the driveTFT.

Forming a bank area around the light-emitting element so that the bankarea overlaps the drive TFT and the control TFT, forming a cut in thebank layer between the light-emitting element and the drive TFT andbetween the light-emitting element and the control TFT provided in thenext pixel, and laying a light-shielding film on the bank layer at leastin a portion thereof near the cut, it is possible to reducelight-induced leakage resulting from the light form the light-emittinglayer being incident on the semiconductor layers of those TFTs andthereby provide a display device with high display quality.

Forming the drive TFT and the control TFT as a-Si TFTs of either ap-channel type or an n-channel type, it is possible to simplify thefabrication process, to eliminate the need for complicated fabricationequipment, and to increase the yield rate and simultaneously reduce thecost.

1. A display device having a plurality of pixels arrayed so as to form amatrix-like pattern, the display device comprising: an organicelectroluminescence element that is formed in a longitudinally oblongshape in each pixel; a drive thin-film transistor that is formed in eachpixel and that feeds a current to the organic electroluminescenceelement to make the organic electroluminescence element emit light; anda control thin-film transistor that controls operation of the drivethin-film transistor, wherein the drive thin-film transistor is formedin a laterally oblong shape, a gate signal line and a source signal lineconnected to the control thin-film transistor are arranged in amatrix-like pattern, the organic electroluminescence element is arrangedso that a longer-side direction thereof is parallel to the source signalline, the drive thin-film transistor has a channel region formed in anelongate shape, the channel region being arranged so that a longer-sidedirection thereof is parallel to the gate signal line and to shortersides of the organic electroluminescence element, and being disposedover a substantially entire area of each pixel along the gate signalline, and the drive thin-film transistor and the control thin-filmtransistor have a semiconductor layer formed of non-crystal silicon. 2.A display device as claimed in claim 1, wherein the drive thin-filmtransistor and the control thin-film transistor are formed of amorphoussilicon.
 3. A display device as claimed in claim 1, wherein of a sourceelectrode and a drain electrode of the drive thin-film transistor, oneis formed in a rectilinear shape and the other is formed in a shapesurrounding the one.
 4. A display device as claimed in claim 1, whereinthe drive thin-film transistor has a U-shaped source electrode and adrain electrode located between two fork-like portions of the U-shapedsource electrode.
 5. A display device as claimed in claim 1, wherein foreach row of the matrix-like pattern, a gate signal line is connected togate electrodes of all control thin-film transistors in pixels locatedin the row, and a power feed line is provided for feeding a current viathe drive thin-film transistors to the organic electroluminescenceelements in the row, for each column of the matrix-like pattern isformed a source signal line that is connected to source electrodes ofall control thin-film transistors in pixels located in the column andthat crosses the gate signal line, and within each area surrounded bygate signal lines and source signal lines, the organicelectroluminescence element, the drive thin-film transistor, the powerfeed line, and the control thin-film transistor are arranged in thisorder along the source signal line as seen in a plan view.
 6. A displaydevice as claimed in claim 5, wherein between the drive thin-filmtransistor and the control thin-film transistor is formed a holdingcapacitor of which one electrode is shared as the power feed line and ofwhich the other electrode is formed by an auxiliary electrode thatconnects to the drain electrode of the control thin-film transistor, andthe auxiliary electrode is electrically connected to the gate electrodeof the drive thin-film transistor.
 7. A display device as claimed inclaim 5, wherein the display device comprises organicelectroluminescence elements that emit light of different colors, aplurality of power feed lines are formed so as to correspond to light ofthe different colors, the plurality of power feed lines are arrangedbetween the drive thin-film transistor and the control thin-filmtransistor within a same pixel, and the organic electroluminescenceelements are fed with a current from the corresponding power feed lines.8. A display device as claimed in claim 5, wherein the gate signal lineis used as the gate electrode of the control thin-film transistor, andthe control thin-film transistor is formed above the gate signal line.9. A display device as claimed in claim 1, wherein a bank layer isarranged around the organic electroluminescence element, the bank layeris formed so as to overlap the drive thin-film transistor, a cut isformed in the bank layer between the organic electroluminescence elementand the drive thin-film transistor, and a light-shielding film is formedon the bank layer at least in a portion thereof near the cut.
 10. Adisplay device as claimed in claim 1, wherein a bank layer is arrangedaround the organic electroluminescence element, the bank layer is formedso as to overlap the control thin-film transistor, a cut is formed inthe bank layer between the organic electroluminescence element and thecontrol thin-film transistor formed in a next pixel, and alight-shielding film is formed on the bank layer so as to cover the banklayer and the cut.
 11. A display device as claimed in claim 1, wherein abank layer is formed so as to cover the drive thin-film transistor andthe control thin-film transistor, the bank layer has edges thereoflocated between the drive and control thin-film transistors and theorganic electroluminescence element, and a light-shielding film isformed on the bank layer.
 12. A display device as claimed in any one ofclaims 9-11, wherein the display device further comprises: a pixelelectrode that is arranged below a light-emitting layer of the organicelectroluminescence element and that connects to the drive thin-filmtransistor; and a common electrode that is arranged so as to face thepixel electrode with the organic electroluminescence element interposedin between and that covers the bank layer, and the light-shielding filmis formed by the common electrode.
 13. A display device as claimed inany one of claims 1-11, wherein the drive thin-film transistor and thecontrol thin-film transistor are of an n-channel type.
 14. A displaydevice as claimed in any one of claims 1-11, wherein the drive thin-filmtransistor and the control thin-film transistor are of a p-channel type.15. A display device as claimed in any one of claims 1-11, wherein achannel width of the drive thin-film transistor is equal to or more thanthree times the channel width of the control thin-film transistor.